Solvent extraction



Sept. 25, 1945.l w. E. BOOTH ETAL SOLVBNT EXTRAC'IION Filed June 5, 1941 INVENToRs WILLIAM EDWARD BOOTH AND RICHARD c oswAY A TTORNEY Patented Sept. 25,1945

SOLVENT EXTBACTION william saura Booth, anneorn, ma melma Cosway, Penketh, England, assignors to 4Im- Chemleal Industries Limited, a corporaperial tion of Great Britain Application June s, um, serial No. sea-122 l rn Great Britain :une 1o, mc i This invention relates to improvements in solvent extraction, and more particularly to im' provements lin metal degreasing processeswhich include steps i'or recovering solvent from residues obtained in such processes, and to apparatus ".5

therefor.

'Various types of apparatus have been proposed for treating metal and like non-absorbent articles with a volatile liquid iat solvent, or with solvent vapour, wherebyiat and grease are removed -l0 and remain dissolved in the solvent. vA typical apparatus for such a purpose comprises a tank having; in the lower part means for'vaporising solvent, and in the upper. part 'cooling coils` around the periphery to condense solvent val5 pours. Normally there is provided immediate-v ly beneath the coils a shallowv trough in which f condensate can be collected and either drawn oil,

e. g., to a storetank, or allowed to overilow the part of the tank to be revaporlsed. Other solvent extraction processes are known in which volatile solvents are used to extract or remove constituents from the goods treated, as for example,

V In the case of the solutions obtained by' any olf-these processes, most of the solvent can usually be recovered without diillculty by simple dis- "buaed against decomposition by the addition of small amounts'ot triethylamine, the distillate will form two layers, one of solvent and one of water, and what amine does distil will be distributed between the -two layers, and on this account the solvent will be deprived of stabilizer.

According to the present invention a solvent extraction process in which a water-immiscible volatile solvent is used and in which thesolvent isrecovered from residue containing a high proportion of constituents other than solvent includes the steps of adding to such residue-an amount of water considerably less than that lluill-l 1 cient for the steam distillation oi all the solvent therein, submitting the water-containing residue Y to indirect heating to distil oil water and some of the solvent, allowing the 'distillate to separate into lan aqueous layer and a non-aqueous layer of solvent, returning the aqueous layer to the still conrim of the trough and ilow back into the loweriio taining the residue while continuing the heating `to distil oi! further quantities oi solvent and separately withdrawing the solvent layer. By indirect heating wemean, e. g., gas or electrical heating Y v where the material heated is separated from the' dry cleaning, degreasing wool or textiles, or ex-'sc trscting seeds, ilsh livers, or bean meal.

heating medium by a heat-conducting' surface, as distinct from heating by the introduction o f'liv'e ,4

when the volatile solventi chlorinated hy-l drocarbon stabilized with an organic compound tillation, but the removal of the last portions oi 80 whining n basic nitrogen atom, e. g., pyridine solvent often presents considerable difiic'ulty on account of the high temperature which is necessary for their 'volatilisation In many cases such high temperatures are to be avoided because oi the tendency of the solvent, or possibly of the solute, to decompose at such temperatures. Thus in using trichloroethylene to remove oil from the surfaces of metal articles and distilling the solution afterwards to recover trichloroethylene, the

oil solution may be heated to between 120 C. and o 130 C. without removing all the solvent, and at higher temperatures there is a tendency for the trlchloroethylene to decompose. A customary method of removing these last portions of solvent is to divert the undistilled residue containing, say

lill-'10% solvent, to a separate still and' to remove the solvent by steam distillation. This method is only applicable where a source oi' steam is available, and, moreover, has the ldisadvantage that when a solvent is used which contains as 50 a stabilizer a small amount of a water-soluble substance, the latter may become separated trom the solvent, and the latter4 then loses its stability. Thus, when using a chlorinated hydrocarbon such as trichloroethylene which has been staor triethylamine, we may also add to the residues formed through decomposition of the solvent during yuse is decomposed to liberate the free' organic base which can be distilled over with the solvent. In some forms oi our invention, as for example in dry cleaning processes. or in rnetltl degreasing with baths o! liquid solvent, the solvent contain-,-

'ing relatively small amounts of impurities which are, however, big enough to prevent it being used again without lpurification. can ilrst be submitted to direct distillation until the residuein the still contains, say 50-'l0% of.. solvent, and is then treated by the method oi' our invention, either-in 4the same or a different still. In other forms of I our invention this preliminary concentration may be unnecessary. Thus, for example, in a vapour type or a liquor-vapour type of metal degreasing apparatus, there will accumulate in the sump o i the vapour compartment a residue containing a u relatively small amount oi solvent, and this resicommunication with the water separator. w ternatively, the water separator may be inside' 2l l due maybe treated directly according to our in-v vention, by diverting it to a special compartment or to a. separate'v still. A small amount o! water is added, and, in the case oi' a chlorinated solvent stabiiised as above described, preferably also a'A 5 small amount of an alkaline material such as sodium carbonate, and the distillation and resolution of the distillate into aqueous and non-aqueous portions is then carried out as described above.

'I'he treatment may becarried out either continuously or intermittently according to the rate at which the residue from the extraction process accumulates. Conveniently the distillate containing water and solvent is delivered to a water separator from which the water can be tapped ci! continually and returned to the still, while the solvent is returnedv to the extraction either directly, or via a stock tank.

Usually we ilnd it suitable to use an amount oi' water weighing up to about a filth of the weight ot the solvent inthe residue, but ya smaller proportion, e. g. al tenth, or a larger proportion, e. g. a third, may be used if desired. Prefer- DI'OUGSB,

ably the amount is suilicient to leave the heat 25 ins surfaces covered when all the solvent has been removed. v

According to a further feature of our invention we provide an apparatus for carrying out the above process comprising a vessel having means for vaporising solvent in the lower part by indirect heating, and in the upper part means,- e. g., condensing coils for condensing solvent vapour, a water separator to receivev the condensate so formed, means to withdraw water from the separator and returnit to the said vessel, and means to withdraw the solvent. suitably the vessel is incorporated in the apparatus for carrying out .the solvent extraction process, and

means are provided for returning solvent from to the water separator directly or indirectly, e. g., via a stock tank, to the extracting compartments oi' the apparatus. Y

Ii desired, the water separator may be external to the vaporiser, and operatively connected to it u by suitable pipes, a trough being provided immediately beneath the condensing means to collect the condensate and deliver it to the piping in A1.

the lvessel and in direct communication with the trough. Again. the trough itself may be adaptedto eil'ect vthe separation -of the condensate into an aqueous layer and a solvent layer, the trough being then provided with means whereby `the aqueous layer can be returned to the lower part oi' the vessel. and means whereby the solvent layer can be transferred to a store tank, or to the solvent extraction compartments as may be de'- `sired. Conveniently it is arranged that the lower 00 layer is drawn o!! through a constant level device which maintains the level of the lower layer in the trough while the upper layer is run on immediately above that level. When the apparatus is intended for use with volatile solvents such as trichloroethylene. which are denser than water, the -lower layer will be the one which is delivered to a store tank or solvent extraction apparatus, and the upper aqueous layer can be allowed to iow back into the vaporizer through an overilow at the appropriate level.

Thus the apparatus, as adapted 'i'or use with a solvent denser than water, may take the iorm of a closed tank having steam coils or gas burners lor heating the lower part of the tank, while assasec inside. suspended from the top or round the uP- per part of the walls o! the tank, are condensing coils through which cold water or brine can be circulated. Immediately beneath the coils is a relatively deep trough arranged to catch condensate falling from the coils 'and constructed with a sloping bottom. From near the lowest -part oi' the trough a pipe passes through the wall of the tank in a vapour-tight manner toone leg of an inverted U-tube, the bend of which U is somewhat below the level of the top ot the` trough, and iromvwhich rises a vent pipe extending well above it. The other leg of the U tube isconnected to the store tank or solvent extraction apparatusl to which distilled solvent is tube delivered. Slightly above the level of the bend of the U tube the trough is provided with an overilow through which liquid can now back into the tank.

20 In using this apparatus ythe solvent-containingl residue and an appropriate amount oi' water are charged into the tank, cooling water is supplied to the conde coils, and heating of the charge is begun. Steam distillation of the solvent commences, water and solvent condense on the coils, and the condensate falls into the trough where a separation into layers takes place. When sumcient condensate has collected the lower layer, consisting of the solvent, runs oil through the liquid in the trough, but the vent pipe prevents a siphcn being set up which would emtpy the trough of both solvent and aqueous layer. Instead, the

latter accumulates in the upper 'part oi the trough until it reaches the level o! the overow through which it then hows and drops back into the charge to serve in the steam distillation of more solvent.

When a special receptacle within the vessel is used 'as a water separator a similar siphon arrangement of piping can be employed to withdraw the layers from it and return them to the store the solvent recovery process while with the cocks in the reverse positions the apparatus will act as a degreasing apparatus in which any water which is introduced into it on the goods being treated or the plant. while solvent is returned to the plant vto maintain the supply oi vapour for degrees ur invention may also be incorporated very conveniently in a metal degreasing apparatus oi'- the type comprising an open-topped vessel, of which the lower part is divided into a number of compartments for holding liquid solvent and one for generating solvent vapour, and in which a circulation of solvent is set up through the various liquid solvent compartments into the vapour generating compartment, while the vapour F formed therein is condensed on condensing coils 7" in the upper part of the vessel and is returned inverted U tube as the result oi' the head or otherwise, will be automatically rejected and thus prevented from accumulating in e when this compartment is not I, also extending assess;

partment isoperated. Thisaccumulation is delivered, either continuously or intermittently, to

is provided with an extra the special compartment through appropriate piping, a suitable amount of watiler is then added,

Aand the solvent recovered Inj normal operation, the treatment of the residues will only need to be carried out intermittently, and the times in use'will provide the opportunity for removing the operation.

'The accompanying diagram represents a sectional elevation of apparatus suitable both for degreasing articles with solvent vapour'and for recovering solvent from solvent vent being denser than water.

In thisv diagram the tom l forming a sump for holding liquid solvent or solvent-containing residues, and beneath it is a gas burner 2 by which they may- Around the perimeter of the upper walls of the tank are condensing coils 4 which can be provided with cooling water by means not shown, and just beneath them is a narrowslopingtrough round the perimeter of the walls of the tank, lto catch condensate dripping from the coils. At the lowest level of tiie'troush is an orifice through which condensatecan flow into receptacle I and thence through the piping shown to water separator 1. Liquid can be tapped oft from water separator 1 at two places, the one through piping indicated generally by- I3 by which water, the lighter liquid, can b e withdrawn, and the other through piping indicated generally by I4,' by which solvent, the denser liquid, can 'be withdrawn. Pipes Il and I4 reach downwards approximately to the level of the solvent sump in the tank and there join pipe I! by which'liquid can be returned tothe sump at inlet I2. Inlet I2 is situated somewhat above the Ylevel oi pipe Il so that a liquid seal is formed in that pipe preventing vapour passing alongv it. Between water separator 'I and pipe Il, pipes I3 and I4 are fitted with valves 9 and Il respectively to control the ilow of liquids in the pipes, and between water separator 1 and these valves sand Il are branch pipes fittedwlth valves l and I respectively by which liquid can be drawn ofi. suitably the branch pipe and valve III on the solvent pipe I4 communicate with a store tank, while branch pipe and valve U on the water line Il enable water to be delivered to a drain. The

substantially solvent-free oil or grease produced during its 2 separator 1,

s collect in water separator I and seperate into layers. The water layer will be returned to thesump vla valve 0, pipe I3 and pipe Il, while the solvent layer will be discharged to a store tank or drumby valve Ill. .Ultimately this discharge of solvent'will cease, indicating that recovery is complete.

The apparatus may also removed to allow the introduction or withdrawal of the goods to be treated, and into the tank I,-either almost up to the level of receptacle I if it is to be Aused for hot liquid solvent degreasing, or merely suiiicient to fili the sump if it is to be used for vapour degreasing. Qn supplying cooling water to coils 4 and lighting burner 2, the apparatus will be ready for operation; solvent vapours 'on coils 4 and be returned to the sump through val',e II and pipes Itand Il, while any water adventitiously introduced will aisovaresidues, the sqltank I vhas a sloping botporize, be condensed on coils 4 and travel to separator 1, where it will be rejected through valve l.

The following is an example or the manner in which the process of our inventioncan be employed. All parts are by weight.

Example A liquor-vapour metal degreasingapparatus was vprovided with a subsidiary still to which be heated.-

residues from the sump of the vapour compartirom which the water layer cduld be returned to the still and the solvent layer to the liquor:

compartment o! the metal degreaser. The plant was gas-heated and fitted with a thermostat.

whereby'the heating ceased when the temperature exceeded 120 was charged with trichloroethylene and used t0 degrease metal articles .contaminated with heayy apparatus is also provided with a removable lid I.

and I0 are opened and valves 8 of water and heating so residue was recovered.

oil. As oil accumulated in Vthe sump of the vathe mixture of oil and solvent pour compartment i! into the subsidiary still was gradually tapped until 230 -parts had. collected. This con B3 parts of oil, the rest being trichloroethylene. This batch was thendistilled by adding 30 parts that the distillate collected in the water separator, from which the water layer waslreturned to the still to serve inthe distillation of more of the solvent, and the solvent layer was returned to the liquor ccmpartment. When the distillation process ended 95.4% of rhemenicroethyiene originally in the For comparative lpurposes a similar batch was distilled by the gas heating alone, i. e., without the solvent' was recovered when' the thermostat cut oi! the supply of heat; the remainder o! the 'I solvent was left in the stili.

We claim:

l. In a process for recovering solvent in the solvent-containing residue from a solvent extraction process employing as solvent a volatile chlorinated hydrocarbon stabilized with a water-soluble organic compound containing a basic nitrogen atom. said residue containing a high proportion of constituents other than solvent. the steps of adding water to the said residues in an amount not -exceeding a ilfth part by weight o! the solvent in the residues, submitting the water-containing residue to indirect heating to distil o! water and some o! the solvent, allowingthe distillate to sepbe lused as e solvent .vapour or as a hot liquid solvent degreasing apparatus. For this purpose valves l and II are opened and valves s and, le are closed. lid II is solvent is charged v formed will condense delivered,y and a water'separator i to-receive the distillate from the subsidiary still C. The degreasing apparatusl tained` adding any water. '19% of t l V y 2,385,564 4 4. In a process lor lrecovering a volatile grease srate into sn aqueous layer and a non-aqueous layer of solvent, returning the aqueous layer to the undistilled residues while continuing the heating to distil oli further quantities oi solvent, and

separatelywithdrawing the solvent layer.

2. In a process tor recovering solvent in the solvent-containing residues vfrom a solvent extraction process employing as solvent a volatile chlorinated hydrocarbon stabilized with triethylamine, said residuescontaining a high proportion lo of constituents other than solvent. the steps ot adding to the said residues a small quantity ot a non-volatile basic material and an amount oi' `water considerably less than that sumcient for I the steam distillation of all the solvent therein, submitting the water-containing residue to indirectheating to distil 'oil' water and some of the solvent, allowing the distillate to separate intoV an aqueous layer and a non-aqueous layer of solvent, returning the'aqueous layer to the undistilled res.. idues while continuing the heating to distil oi! turther quantities of solvent, and separately with-:

. drawing. the solvent layer.

' 3. In a process for recovering a volatile srease solvent in residues from a solvent extraction proc- 1 ess employing a volatile chiorinated'hydrocarbon A solvent immiscible with waterand stabilized with a water-soluble organic compound containing a basic nitrogen atom. which residues contain' aA high proportion of solvent. the step which comprises distiliing solvent from the said residues so as to leave concentrated solvent-containing residues, adding to the said concentrated residues la non-volatile basic material and a quantity of water considerably less than that suilicient for 3' the steam distillation of all the solvent therein.

submitting the water-containing residue to indi-l rect "heating to distill o3 water and some of thesolvent, allowing the distillate to separate into an aqueous layer and a non-aqueous layer of solvent, i0

lreturning the aqueous layer to the undistilled residues while continuing the heating to distil ori further quantities of solvent, and separately withdrawing the solvent layer. v

solvent in the residues from a solventextraction process employing a volatile chlorinated hydro carbon solvent immiscible with water and stabilized with a. water-soluble organic compound containing a basic nitrogen atom, which residues contain a high proportion of solvent, the step which comprises distilling solvent from the said residues so as to leave concentrated residues containing between 50 per cent and 70 per cent oi' solvent. adding to the concentrated residues a.

non-volatile basic material and a quantity ot .water considerably less than that suillcient i'or 'the steam distillation of all thesolvent therein,

l action of a volatile chlorinated hydrocarbon solvent immiscible with water and stabilized with a water-soluble organic compound containing a basic nitrogen atom, whereby solvent contaminated' withnon-volatile constituents is produced, the A steps of distilling the contaminated solvent to give purined solvent and solvent-containing residues, adding to said solvent-containing residues a nonvolatile basic material and an amount oi' water considerably less than that suilicient for vthe steamv distillation of all the solvent in the residues, submitting the water-containing residues to indirect heating to dlstil oil.' water and some of the solvent,

' allowing the distillate to separate into an aqueous and a non-aqueous layer, and returning the aque-- 'ous layerto the undistilled residues while continuing the heatingso as to distil olf further quanp tities of solvent.

v WILLIAM EDWARD BOO RICHARD COSWAY. 

